These studies are concerned with the cell and molecular mechanisms which lead to cell and tissue destructive reactions mediated by lymphoid cells from man and experimental animals in vitro. While these are complex reactions, there apear to be three common steps: a) "recognition" - promoted by lymphocyte and target cell membrane interaction; which results in b) "activation" of the lymphocyte to become the effector cell; and c) "lysis" of the target cell, which can be either rapid or delayed. Our studies have been predominantly directed toward understanding, at the molecular level, the mechanism(s) involved in cell lysis. Human lymphocytes, once activated, can release substances in vitro, which are growth inhibitory or toxic for cells. Physical-chemical studies reveal that human LT molecules are proteins and can be separated into classes and subclasses on the basis of physical-chemical properties. Physical and immunologic studies reveal these molecules complex with each other to assume different molecular weight. A large form, termed complex, has been identified, which is associated with a specific antigen binding receptor(s). This latter finding is highly significant, for it indicates how nonspecific lytic molecules can be focused to induce specific lysis. Moreover, these and other data indicate the large MW complex is more active than the individual LT subunits. A new breakthrough in LT production allows similar studies in experimental animals. Studies with anti-LT sera provided definitive proof that LT molecules are lytic effectors in lectin induced and immune target cell lysis in vitro. However, immune lysis may involve LT in the complex form and may only be operative in the microenvironment between aggressor and target cells. Various types of experiments are underway to examine the role of these materials in vivo.